1. Field of the Invention
The invention relates to energy-absorbing shock cells for mounting bumper devices on outwardly downwardly slanting legs of offshore oil rig platforms, barge loading docks or similar structures. More particularly the invention relates to a new method of manufacturing such a shock cell which has inner and outer steel pipe members with an intervening rubber sleeve bonded with adhesives and vulcanization around and to the inner end portion of the inner pipe member and within and to the outer end portion of the outer pipe member with the sleeve rubber under extreme compression between the pipe members capable of absorbing energy by deflection of the rubber when external force is applied to the outer end of the inner pipe member.
In addition, the invention relates to a method of making such a shock cell without restrictions as to the length of the outer pipe member so as to enable shock cells having different outer pipe member length requirements to be used to mount a bumper device on a platform leg extending angularly downwardly outwardly into offshore water, without an outer metal pipe section having to be welded to the outer shock cell pipe member to obtain the necessary length of shock cell outer pipe member required when mounting a bumper device on an angularly extending platform leg.
2. Description of the Prior Art
Shock cells have been used traditionally to form the upper mount or the upper and lower mounts for vertical members mounted thereby on angularly extending platform legs between which vertical members bumper devices are supported. Sometimes the vertical members, themselves act as bumpers being surrounded by rotatable rubber tires or other resilient members or rings.
These traditional shock cells known in the art comprise inner and outer cylindrical metal pipe members and a rubber bushing or sleeve therebetween bonded or cemented to the inner end of the inner pipe member and to the outer end of the outer pipe member. The free end of the outer pipe member usually is welded or otherwise mounted on one of the legs of an oil rig platform structure.
When two spaced shock cells are used to support a vertical bumper member on an angularly extending platform leg, the upper shock cell must have an outer pipe member longer than that of the lower shock cell. It has not been believed to be economically feasible to manufacture such traditional shock cells with numerous different length outer pipe members. This is because such different length outer pipe member products in accordance with past manufacturing practice each requires its own mold wherein the cavity must receive the two pipe members for bonding the rubber sleeve to the inner and outer pipe members by vulcanization.
This situation in the past has required a standard sized shock cell, when used as the upper mount for a vertical bumper device, to have a pipe section welded to the shock cell outer pipe member to obtain the required length of outer cell pipe member to satisfy the angular location of the platform leg on which the shock cell is mounted as well as the length of the vertical bumper member.
Such welded extension joints are quite visible when inspecting offshore oil rig installations; and such welded joints may be objectionable and may be the location of failure of the shock cell when a ship or barge bumps the bumper device with extreme force, either during ship headway movement or during up and down movement of the ship against the bumper as a result of wave action.
The traditional prior art shock cell has another objectionable characteristic resulting from its manufacture using usual rubber molding and vulcanizing procedures to bond the rubber sleeve located between the outer end of the outer pipe member and the inner end of the inner pipe member to both members at one time in one operation.
In such usual procedure, the inner and outer pipe members are placed and held in a suitable mold cavity with the inner end of the inner pipe member extending or telescoped coaxially into and overlapped by the outer end of the outer larger diameter pipe member. A sufficient quantity of uncured rubber is supplied between the telescoped ends of the two pipe members. Then the mold is closed and heated to carry out a usual vulcanizing operation to bond the rubber to the ends of the two pipe members under suitable temperature and time conditions for the particular rubber composition being vulcanized.
The shock cell pipe members bonded to and joined by the vulcanized rubber sleeve is removed from the mold. During cooling, the rubber bonded to the two pipe members contracts or shrinks somewhat with the result that the rubber body between the pipe member ends has a condition of tension therein and the bonds between the rubber body and pipe members ends also are under tension.
Such prior shock cell absorbs energy upon deflection of the rubber when the inner member is forcefully telescoped into the outer member until the rubber fails in tension at one of the bonds between the rubber body and pipe members or in shear within the tensioned body. The residual force after shock cell failure is transmitted directly to the platform structure.
Various types of such traditional shock cells and bumpers mounted thereby are shown in U.S. Pat. Nos. 3,991,582, 4,005,672, 4,098,211 and 4,109,474.
Accordingly, there is an existing need and long-standing want in mounting bumper devices on oil rig structures for a shock cell which eliminates welded joints in the outer pipe member of such cells; and for a shock cell which has the ability to absorb energy by substantially increased deflection of the rubber sleeve while transmitting a smaller residual force to the platform structure when the bumper mounted on the shock cell is struck severely by a vessel, as compared with smaller rubber deflection and larger residual force transmitted to the platform structure characterizing prior devices.